Controlling induced seismicity during hydraulic stimulation of a 6 km deep Enhanced Geothermal System in Finland.

We show that near-realtime seismic monitoring of fluid injection allowed control of inducedearthquakes during the stimulation of a geothermal well near Helsinki, Finland. The injectionwell, OTN3, was drilled down to 6.1 km-depth into Precambrian crystalline rocks. WellOTN3 was deviated 45∘ from vertical and an open hole section at the bottom was divided intoseveral injection intervals. A total of 18,159 m3 of fresh water was pumped into crystallinerocks during 49 days in June- and July, 2018. The stimulation was monitored in near-realtime using (1) a 12-level seismometer array at 2.20-2.65 km depth in an observation welllocated 10 m from OTN3 and (2) a 12-station network installed in 0.3-1.15 km deepboreholes surrounding the project site. Earthquakes were processed within a fewminutes and results informed a Traffic Light System (TLS). Using near-realtimeinformation on induced-earthquake rates, lo-cations, magnitudes, and evolutionof seismic and hydraulic energy, pumping was either stopped or varied betweenwellhead-pressures of 60-90 MPa and flow rates of 400-800 l/min. This procedure avoidedthe nucleation of a project-stopping red alert at magnitude M2.1 induced earthquake, alimit set by the TLS and local authorities. The stimulation resulted in detection of>43,000 earthquakes with -1.2<ML<1.9. The original catalog was relocated usingdouble-difference technique to improve hypocenter precision. The 4032 relocatedearthquakes were used to investigate the spatio-temporal evolution of seismicity, seismicenergy release, and maximum magnitude in response to injection. We found hypocenterdistribution, Gutenberg-Richter (GR) distribution and relation between hydraulic andradiated energy suggest (re-)activation of size limited network of distributed fractures.The temporal behavior of G-R b-value, as well as a lack of temporal (Omori-type)correlations in a presence of spatial localization of earthquakes suggest very limitedearthquake triggering and stress transfer at low level of ambient stress. The maximumobserved magnitudes scale with stored elastic (=hydraulic) energy, following afracture-mechanics based model of Galis et al. (2017). Our results suggest a possiblephysics-based approach to controlling stimu-lation induced seismicity in geothermalprojects. [ABSTRACT FROM AUTHOR]